Synthesis and characterization of a new copper-based polyoxomolybdate and its catalytic activity for azide-alkyne cycloaddition reaction under UV light irradiation

A new organic-functionalized Cu-based Anderson-type polyoxomolybdate, namely (C7H15N4)2[Na(H2O)4]2[C6H12CuMo6N2O24]·2(H2O) (CuII-POM), was synthesized via a simple one-pot reaction and subsequently characterized using a range of analytical and spectral techniques. Structural investigation by single crystal X-ray diffraction analysis revealed that the polyanion component of the synthesized compound (i.e. [C6H12CuMo6N2O24]4−) possesses a δ-isomer Anderson-type structure, which is surrounded by four lattice water molecules and four [C7H15N4–NaH15(H2O)8]4+ cations in the crystal packing arrangement. The resulting double-sided tris-functionalized Anderson-type compound can function as highly effective heterogeneous photocatalysts for the copper(I)-catalyzed Huisgen azide-alkyne cycloaddition (Cu-AAC) reaction of terminal alkyne, benzyl halides, and sodium azide (acts as the azidonation and reducing agent) in aqueous media. Ultraviolet light irradiation enhances the catalytic activity of CuII-POM ~ 4.4 times of the “off” situation under reaction conditions of 0.00239 mmol cat., 80 °C, 8 h, 2 mL H2O, So that the isolated yields for the AAC reaction involving a variety of terminal alkynes and benzyl halides using the CuII-POM catalyst ranged between 19–97%. The current study is the first report about using an efficient and economical Cu(II)-POM/UV/NaN3 catalytic system in the Cu-AAC reaction and reveals its significant potential for applying to other Cu(I)-catalyzed reactions.

band of POM to the visible light region, most of these compounds become active under ultraviolet (UV) light irradiation [29][30][31][32] .As extensively detailed in former literature, UV light irradiation on POMs causes an oxygento-metal charge transfer (O → M CT) as a result of promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).The resulting photo-excited POMs, known as POMs*, are potent oxidizing compounds and are able to form POM − through the abduction of an electron from proper reducing compounds 27,33 .The as-prepared POM − has the ability to deliver its electrons to various chemical species, for instance, metal ions.
Functionalization of POM with organic ligands is an appropriate method to extend the structure of this group of compounds and control their electronic properties, stability, and compatibility with organic media 34,35 .Among the various synthetic strategies carried out in this field (organoalkoxylation, organoimidization, organoarsonylation, organosilylation, organophosphonylation and organotin), unique interest has been paid to the covalent attachment of organic moieties to the POM surface in direct mode (organoimidization and organoalkylation) 34,36 .Apart from other advantages of the organoalkylation strategy (e.g. more effective synthesis), the greater stability of organoalkylated POMs in aqueous media compared to organoimidizated counterparts can make them susceptible to use in water-active catalytic systems 36,37 .Anderson-type POMs are one of the well-studied compounds for grafting strategies with a wide variety of multidentate alkoxy ligands (such as tris-ligands; RC(CH 2 OH) 3 , R = -NH 2 , -OH, -CH 3 etc.).To date, numerous isomers of single-side and double-side tris-functionalized structures have been prepared and reported from them [38][39][40][41][42] .The δ-isomer of Anderson-type POM is one of the trisfunctionalized Anderson derivatives, in which triol ligands replaced all three μ 3 -OH oxygen atoms at one sides of the flat POM cluster.These compounds are typically produced in several complex steps, and alternative and attractive one-step production methods require further research.
The Cu(I)-catalyzed Huisgen cycloaddition of azides and alkynes, also known as Cu-AAC, is the most wellknown click reaction that produces 1,2,3-triazoles gently and selectively.Triazoles are relatively stable compounds with numerous applications in medicine, organic chemistry, and bioregional synthesis [43][44][45][46] .Furthermore, triazole derivatives such as fluconazole have been synthesized and designed as antifungal agents 47 .Thus far, various catalytic systems containing a wide range of copper species, such as Cu(II) compounds with a reducing agent (typically sodium ascorbate) or Cu(I) compounds, have been developed for catalyzing AAC reactions 48,49 .Lately, the photo-chemically conducted Cu-AAC reactions under UV light irradiation (the strategy of in situ generation of Cu I from a Cu II using light) have demonstrated their potential as a dependable approach for diverse triazoles synthesis applications [50][51][52] .Cu(I) generation during the light-induced Cu-catalyzed reactions can be accomplished by either directly exposing Cu(II) compounds to UV and vis irradiation or indirectly reducing Cu(II) using a photosensitizer 51 .In the direct photoreduction of Cu(II) centers, promoting an intramolecular electron transfer from the ligand's π-system to the Cu(II) center and forming Cu(I) species occurs through UV light absorption.In the commonly used indirect strategy, the photoinitiator first absorbs ultraviolet light and forms a reaction intermediate, which can then promote the photoreduction of Cu(II) to Cu(I) species 53,54 .Nevertheless, to our knowledge, there is no report on utilizing UV light-assisted photocatalytic Cu-AAC reaction based on Andersontype POMs, where no supplementary reducing agent or photoinitiator is needed.
Here, a new Cu-based δ-isomer Anderson-type polyoxometalate (Cu II -POM) was synthesized, and after fully structural characterization, it was used as an efficient catalyst in the Cu-AAC reaction to prepare target 1,2,3-triazoles under UV light irradiation without the need for an additional reducing agent.The stability and true heterogeneity of the as-prepared catalyst were also assessed under previously established optimal reaction conditions.5.85 mmol of sodium molybdate dihydrate was added to a mixed solution of 15 mL acetic acid, 2 mL methanol, and 25 mL deionized (DI) water and stirred for 5 min.To this solution, copper nitrate trihydrate (2.06 mmol) and ammonium acetate (12.97 mmol) were added, and the resulting mixture was agitated at 70 °C for a duration of 4 h.Finally, the solution (in an open vial) was positioned in a fixed place for crystal formation.The combined yield of both crystals and bulk powder is 65%.

General procedure for the Cu-AAC under UV light irradiation
Terminal alkyne (0.5 mmol), sodium azide (0.55 mmol), and the organic halide (0.55 mmol) were added to 2 mL H 2 O containing 0.00239 mmol Cu II -POM as a catalyst.Then, the reaction mixture was warmed to 80 °C in an oil bath and stirred under UV light irradiation.Following a specific duration, the resulting mixture was extracted with ethyl acetate (EtOAc; 15 mL), and the gathered organic phase was dried over anhydrous CaCl 2 .After removing the solvent under reduced pressure, it was possible to acquire the target 1,2,3-triazoles without any requirement for a purification step.Ultimately, the obtained product was weighed and the reaction efficiency was calculated.The 1 HNMR technique was used for product validation.

Characterization of Cu II -POM
The double-sided symmetric tris-functionalized Anderson-type POM with a copper(II) heteroatom and the Mo(VI) addenda atoms (Cu II -POM) was prepared via a one-pot reaction protocol.In this facial approach, the addenda atom, the heteroatom, and ammonium acetate salts were dissolved in acetic acid/methanol/ DI water mixed solution and heated at 70 °C for 4 h to produce the desired tris-functionalized δ-isomer Anderson cluster.The resulting compound is sufficiently stable to be preserved in air for several months.Also, the solubility test revealed that the Cu II -POM is insoluble in acetone, ethyl acetate and water but it is barely soluble in ethanol, methanol, and acetonitrile.Single-crystal X-ray analysis uncovered that ) crystallizes in a monoclinic space group I2/m.The detailed crystal structure of Cu II -POM is depicted in Fig. 1.The anion of [C 6 H 12 CuMo 6 O 24 ] 4− possesses the conventional flat Anderson-type polyoxomolybdate structure, consisting of an edge-sharing central heteroatom octahedra unit (CuO 6 ) surrounded by six edge-sharing MoO 6 octahedra units.Two trimethanolamine (tris) ligands, which are produced during the reaction, were attached to the cluster by supplanting six μ 3 -oxygen atoms on either side of the plane to form a δ-isomer of the tris-functionalized Cu-based Anderson-type POM.Whereas the copper atom is connected to six deprotonated μ 3 -oxygens from the tris ligand, each molybdenum atom is encompassed by two terminal oxygens, two μ 2 -bridged oxygens, and two μ 3 -oxygens of the tris ligand.Within the cation of [C 14 H 46 N 8 Na 2 O 8 ] 4+ , each sodium atom is surrounded by one N-methyluroptropine (C 7 H 15 N 4 ) + molecule and five water molecules (two bridged water and three terminal water).The methyl group on the N-methyluroptropine (C7, H7A, H7B, and H7C), a lattice water molecule (O4W, H4WA, and H4WB), the oxygen atom of one terminal water molecule (O3W) and the hydrogen atoms of the C1 and C3 (H1A, H1B, H3A, and H3B) each were found to be disordered over two positions and the site occupancy factors were refined to 0.5 in all cases (turquoise and yellow).The particulars regarding the data collection parameters, crystallographic data, and final agreement parameters have been compiled in Table 1.
In   S2 online lists efficient interactions (hydrogen bonding) for building such a framework structure.
The composition of the prepared POM was also further characterized by the energy-dispersive X-ray spectroscopy (EDX) elemental analyses, inductively coupled plasma optical emission spectrometry (ICP-OES), carbon, hydrogen and nitrogen (CHN) elemental analysis as well as FT-IR spectroscopy.The EDX analysis results confirmed the presence of N, C, O, Cu, and Mo elements in the structure of the prepared Cu II -POM, in accordance with the results of single-crystal X-ray analysis (see Supplementary Fig. S2 online Figure S3 shows the TGA of the Cu II -POM from 25 to 1000 °C.The Cu II -POM showed the initial low-temperature weight loss of 3.8 wt% due to water molecules.Then, the weight loss (21%) beginning at 210 °C is primarily due to the decomposition of N-methyluroptropine (C 7 H 15 N 4 ) + cations, indicating that the Cu II -POM is stable below 210 °C.The weight loss of Cu II -POM with a mild slope occurred between 210 and 650 °C corresponds to the proportion of trimethanolamine (tris) ligands and oxygen molecules contained in the Cu II -POM (19 wt%).
In the FT-IR spectrum of Cu II -POM, the characteristic bands of water, N-methyluroptropine, and tris are observed at 3100-3700 (the broad band with the center at 3500 cm −1 ), 3004, 2924, 2860, 1466, 1255, and 1020 cm −1 resulting from ν O-H of water molecules, ν C-H of CH 3 , ν assym C-H, ν sym C-H, δ C-H of CH 2 , and ν C-N (on tris and N-methyluroptropine), respectively (see Supplementary Fig. S4 online) 55 .Moreover, the peaks located at 901. 828, and 645 cm −1 are related to Mo=O t , Mo-O b -Mo, and Cu-O vibration modes, confirm the successful synthesis of desired POM.Additionally, the powder X-ray diffraction (PXRD) pattern (see Supplementary Fig. S5   20), indicating the same chemical structure with lower crystallinity of the bulk powder.The ultraviolet-visible (UV-Vis) diffuse reflection spectroscopy of the yellowish-green solid sample of Cu II -POM exhibited three strong absorptions in the ultraviolet region (up to approximately 385 nm), indicating its potential application in UV-activated photocatalysis (Fig. 2).

Catalytic effects
Catalytic activity assessment began with the cycloaddition of phenylacetylene (0.5 mmol) to in situ-generated benzyl azide [from sodium azide (0.55 mmol) and benzyl chloride (0.55 mmol)] in the presence of Cu II -POM www.nature.com/scientificreports/catalyst.All the catalytic reactions were conducted under atmospheric and aqueous reaction conditions and direct exposure to UV light irradiation (on/off situation).The optimal conditions were determined by varying the parameters affecting the reaction rate, like solvents, temperature, reaction duration time, and catalyst nature (Table 2).The results showed that without utilizing a catalyst, a control experiment, the intended triazole product cannot be produced even under UV light irradiation (entry 1).Whereas, by loading a small amount of Cu II -POM catalyst (0.00239 mmol) at 25 °C, the yield of the isolated product under UV light enhanced to 63% after 8 h (entry 6).Further enhancement in catalytic efficiency was achieved by raising the reaction temperature, as the molecules' higher velocity increases the frequency of their possible collisions.The maximum product yield was obtained in 8 h at a temperature of 80 °C (entries 6, 9-14) and raising the temperature even more has no impact on the reaction's progress (entry 13).Altering the reaction duration time also had a remarkable influence on the reaction rate (entries 2-6).As shown in Table 2, increasing reaction time from 2 to 8 h raises the yield from 75 to 97% under UV light in "on" situation.Increased reaction duration to 10 h does not enhance the reaction efficiency in either the "on" or "off " states (entry 14).In addition, it was found the efficiency of the reaction was strongly affected by the solvent type (entries 6, 15-18).Several different solvents, including acetonitrile, water, acetylacetone, ethanol, and methanol were evaluated in order to determine the best solvent for the model reaction.No significant product was observed when CH 3 OH, C 2 H 5 OH, and C 3 H 6 OH were used as solvents, while the addition of CH 3 CN and H 2 O enhanced the product yield to 78% and 97%, respectively.The convenient solubility of sodium azide in water is the main reason for this observation.Comparison of the catalytic potential of the asfabricated POM with its parent salts revealed the superior performance of Cu II -POM catalyst at the same reaction  Based on our experimental results and the related literature studies, a possible reaction mechanism is proposed, as depicted in Fig. 3.As mentioned early (see the introduction section), POM compounds have the ability to form an excited state POM* (Cu II -POM* here) with robust oxidizing power as a consequence of charge transfer (CT) from O 2− to Mo 6+ (the O → Mo CT) under exposure to UV light.The photoexcited POM* can abstract electrons from a wide range of compounds, for instance, propan-2-ol, and form a photochemically reduced POM − (Cu II -POM − here), which then reduces its Cu(II) centers through a metal-to-metal CT (Mo V → Cu II ) to create a favorable Cu(I) site for catalyzing the AAC reaction (Cu I -POM formation) 56 .Whereas alcoholic solvents are known as sacrificial reducing reagents in metals photocatalytic reduction and recovery studies 27,28 they are not favorable for our catalytic system, as can be seen from Table 2 (entries 12-14).On the other hand, by conducting a reaction between phenylacetylene benzyl azide and in the absence of NaN 3 , only 15% of the target 1,2,3-triazole was achieved.These results are consistent with former documents about the in situ generation of Cu(I) species in the presence of NaN 3 and disclose both the reducing and azidonation property of the sodium azide in our developed catalytic system 57 .In the continuation of the catalytic cycle, the newly generated Cu(I) sites attack alkyne to produce the Cu(I)-acetylide intermediate, A. Subsequently, the in situ formed azide molecules (from NaN 3 and benzyl chloride reactants) attach to this complex to generate an intermediate complex of B. The nucleophilic attack of nitrogen of the azide to alkyne forms copper(I) metallacycle, which can create the target product (C) after the protonation and elimination of the catalyst.Performing the model reaction in a dark condition (with a yield of 35%) indicated that the reaction could not proceed entirely without UV light irradiation exposure, supporting the proposed mechanism here.
Table 3 illustrates the scope of terminal alkynes and benzyl halides in the AAC reaction under UV light irradiation using the Cu II -POM catalyst.Under optimized reaction conditions (0.00239 mmol cat., 80 °C, 8 h, 2 mL H 2 O, and UV light irradiation), aromatic alkyne of phenylacetylene demonstrated higher reactivity than aliphatic alkynes, and the lowest product yield (19%) belongs to the 2-methyl-3-butyn-2-ol substrate with the highest number of methyl groups (steric effect).The nature of the leaving group on benzyl halide (Cl and Br) does not significantly affect the reactivity.In contrast, the substitution of benzyl chloride with a nitro-withdrawing group at the ortho-position decreased the yield of desired 1,2,3-triazoles.
To reveal the competency of the Cu II -POM catalyst, the AAC reaction results under the optimal reaction conditions in the current study were compared to former studies and summarized in Supplementary Table S3 online.Although our results are comparable to other data presented in this table, using an easy and fast synthesis method (facial one-pot synthesis protocol) without any need for substrate material for providing heterogeneous nature (compared to entries 4 and 5), performing the catalytic reaction in green media (in contrast to entries 2 and 4), low catalyst loading, fewer reaction time (in comparison to 1-2, 5), and no need for additives (like the base and ligand in entries 2-3) are the major advantages of our described catalytic system-indicating the high potential of our designed Cu II -POM catalyst for 1,2,3-triazoles production.Additionally, the photocatalytic approach employed in this study is the novelty of the work, which shows promise for advancing research in this field and generating further interest.
The hot filtration experiment of Cu II -POM has been carried out to certify its actual heterogeneous catalytic activity (Fig. 4).In this regard, the Cu II -POM catalyst was recovered by hot filtration after 6 h, and the catalytic reaction was further performed in the catalyst-free aquatic phase (the filtrate).The results showed that no appreciable enhancement in the cycloaddition efficiency is achieved after 2 h, suggesting no metal active sites leaching and the factual heterogeneity of the as-synthesized catalyst.Furthermore, the recyclability of the as-fabricated Cu II -POM has been checked under the optimized reaction conditions (Fig. 5).For this purpose, after extraction of the desired triazole product (using 2 × 5 mL of EtOAc), the catalyst was isolated from the aqueous phase by centrifugation, washed by water and ethanol, dried at 60 °C, and then used in the subsequent batch of reactions by adding fresh substrates.It was found that the activity of the Cu II -POM catalyst is maintained for two initial cycles, after which a significant drop in activity was observed for two further successive cycles (product yield decreased to 41%).Based on the PXRD and FT-IR findings of the fresh and used photocatalyst (analyzed after three runs), it is evident that a structural transformation to a new inactive crystalline form is the primary cause for this observed phenomenon (see Supplementary Figs.S4 and S5 online).

Conclusion
In conclusion, a facial and economical one-pot protocol was developed for the synthesis of a double-sided trisfunctionalized Anderson-type Cu II -POM catalyst from its parent salts.The as-synthesized catalyst was thoroughly characterized using single crystal X-ray diffraction, PXRD, ICP, CHN, UV-Vis, TGA, EDX and FT-IR analysis, utilized as an efficient catalyst in 1,2,3-triazoles synthesis.The catalytic reaction results demonstrated that our fabricated catalyst is only active when exposed to UV light and performs in water without any requirement for additional reducing agents.Additionally, a hot filtration and recyclability test verified its excellent stability and moderate reusability under applied media.The outcome of this research demonstrates in situ generation of Cu(I) from UV-activated Cu(II)-based POMs is a proper method for one-pot catalytic production of 1,2,3-triazoles.

Figure 1 .
Figure 1.(a) View of the Cu II -POM, with the labelling of the atoms.Thermal ellipsoids are depicted at the 50% probability level, H atoms with arbitrary radii.The site occupancy factors of the two disordered parts were refined to 0.5 (turquoise and yellow).(b) Polyhedral representation of the [C 6 H 12 CuMo 6 N 2 O 24 ] 4− in the crystal structure.

Figure 2 .
Figure 2. Diffuse reflectance spectrum of the as-synthesized Cu II -POM.

Figure 3 .Table 3 .
Figure 3. Proposed mechanism for the AAC reaction in the presence of Cu II -POM under exposure to UV light irradiation.

Table 1 .
Crystal data, data collection, and structure refinement details for Cu II -POM.